Optical storage media are media in which data are stored in an optically readable manner, for example by means of a laser and a photo-detector being integrated within a pickup, the photo-detector being used for detecting the reflected light of the laser beam when reading the data. In the meanwhile a large variety of optical storage media are known, which are operated with different laser wavelength, and which have different sizes for providing storage capacities from below one Gigabyte up to 50 Gigabyte (GB). The formats include read-only formats such as Audio CD and Video DVD, write-once optical media such as CD-R and DVD-R, DVD+R, as well as rewritable formats like CD-RW, DVD-RW and DVD+RW. Digital data are stored on these media along tracks in one or more layers of the media.
The storage medium with the highest data capacity is at present the Blu-Ray disc (BD), which allows to store 50 GB on a dual layer disc. Available Blu-Ray formats are at present read only BD-ROM, re-writable BD-RE and write once BD-R discs. For reading and writing of a Blu-Ray disc an optical pickup with a laser wavelength of 405 nm is used. On the Blu-Ray disc a track pitch of 320 nm and a mark length from 2 T to 8 T+9 T is used, where T is the channel bit length, and which corresponds with a minimum mark length of 138-160 nm. The re-writable BD-RE disc is based on a phase change technology comprising a phase change layer, which uses for example a compound of AgInSbTe or GeSbTe. Further information about the Blu-Ray disc system is available for example from the Blu-Ray group via internet: www.blu-raydisc.com.
New optical storage media with a super resolution near-field structure (Super-RENS) offer the possibility to increase the data density of the optical storage medium by a factor of four in one dimension as compared with the Blu-Ray disc. This is possible by a so-called Super-RENS structure, which is placed above a data layer of the optical storage medium, and which significantly reduces the effective size of a light spot used for reading from or writing to the optical storage medium. The super resolution layer is also called a mask layer because it is arranged above the data layer and only the high intensity center part of a laser beam can penetrate the mask layer.
The Super-RENS effect allows to record and read data stored in marks of an optical disc, which have a size below the resolution limit of a laser beam used for reading or writing the data on the disc. As known, the diffraction limit of the resolution of the laser beam is lambda/(2*NA), where lambda is the laser wavelength and NA the numerical aperture of the objective lens of the optical pickup.
A “Super-resolution and frequency dependent-efficiency of near-field optical disks with silver nanoparticles” is described by Ng and Liu, Optics Express, Vol. 13, No. 23, 14 Nov. 2005, p. 9422-9430. As described, silver nanoparticles in an AgOx layer generate near fields exhibiting a strongly local field enhancement around the nanoparticles due to localized surface plasmons. Sub-wavelength recording marks smaller than lambda/10 were distinguishable since the metallic nanoparticles transferred evanescent waves to detectible signals in the far field. The super-RENS disc studied by Ng and Liu includes a GeSbTe phase change material as a recording layer, an AgOx layer including a random distribution of silver nanoparticles imbedded in the AgOx layer, and two ZnS—SiO2 dielectric layers as protective layers for the AgOx layer.
In US 2005/0009260 a recordable optical disk is described, which comprises a phase change layer or a metal layer as the data layer. As the mask layer a metal oxide layer or a layer of nanoparticles may be used, for example a layer with platin nanoparticles.
In EP 1724768 an optical storage medium is described which comprises a light-transmissible composite layer for providing a super-resolution effect, the composite layer containing nanoparticles with a high refractive index for increasing the refractive index of the composite layer. The composite layer may be applied by using spin coating.
A mask layer for a high-density near-field optical storage system, wherein the mask layer includes nanoparticles embedded in a nonlinear optical material to modify an index of refraction, is disclosed in WO 2005/098843.